1. Field of the Invention
The present invention relates to a method and arrangement of implementing initial antenna beam pointing in a satellite mobile communications system, and more specifically to such a method and arrangement via which the initial antenna beam pointing is very rapidly carried out as compared with a known technique.
2. Description of the Prior Art
If a mobile unit such as an automobile, ship or the like, restarts communications with a stationary satellite, it is necessary to correctly direct the mobile unit mounted antenna beam to the satellite. After completing such an antenna beam point, a usual satellite tracking is implemented.
According to a known technique, the initial antenna beam pointing is carried out by rotating the antenna in azimuth at a somewhat slow speed so as to demodulate incoming signals and subsequently determine if the mobile unit is synchronized with the transmitter (viz., the satellite). A transmission bit rate in an ordinary satellite mobile communications system is as low as several thousands bps (bits per second), and the demodulation and the sync determination require several thousands bps. Accordingly, about one second is needed for completing one cycle of the demodulation and the syn determination. On the other hand, the antenna acquires the signals within a half-power beam width of about 10.degree. (for example) in azimuth plane. This implies the antenna rotating speed should be set to 10.degree./sec. Thus, the antenna takes about 36 seconds until completing one revolution.
Consequently, the above mentioned prior art has encountered the problem in that it takes undesirably a long time until the antenna is correctly pointed to a satellite before restarting communications with the satellite.